1. Field of the Invention
The present invention relates to an image reading apparatus, and in particular an image reading apparatus capable of reading an original image while correcting a shading phenomenon.
2. Related Background Art
FIG. 1 shows an image reading apparatus employed in facsimile, copying machine or the like.
An unrepresented original document, transported by feed rollers 1, 1', 4, 4' along an original guide 3 is illuminated by a light source 2, composed for example of a fluorescent lamp. An image of the original, corresponding to a main scanning line, is guided by a mirror 7 and is focused, by a lens 9, onto a linear image sensor 11 comopsed for example of a CCD. The linear image sensor 11 effects photoelectric conversion and releases image signals corresponding to said image to a signal line 11a in the unit of a main scanning line, as shown by an upper curve in a wave from the chart shown in FIG. 2. The linear image sensor 11 is driven by control clock signals supplied from a reading control unit 23 composed of a microprocessor.
The output image signals from the linear image sensor 11 contain so-called shading distortion, a distortion caused by a shading phenomenon, or an uneven light intensity distribution, of the optical system. Such shading distortion undesirably affects the image signals obtained by image reading, and, in order to eliminate such distortion, the output image signals of the image sensor 11 are supplied to a programmable gain amplifier 13 of which the gain is controllable. To there amplifier 13 there is connected a read-only memory (ROM) 15 which stores information for controlling the gain of the amplifier 13 for compensating for the shading distortion, measured by reading a white reference plate 5 placed in the original reading position, at the manufacture of the image reading apparatus. Said control information is supplied from the ROM 15 to the amplifier 13 under the control of the reading control unit 23 to regulate the gain of the amplifier 13 for each bit of the input image signals, thereby eliminating the shading distortion from said signals. The image signals freed from the distortion are supplied to a signal line 13a as shown by a lower curve in FIG. 2 and to the inverted input terminal of a comparator 19. The non-inverted input terminal of said comparator 19 receives a threshold voltage obtained by dividing a peak voltage of the image signals, retained in a peak-hold circuit 17 connected to the signal line 13a, by a variable resistor 21 connected to the output line of said circuit. The comparator 19 compares said threshold level with the image signal to binary digitize said image signals, and thus binary digitized image signals are supplied to a signal line 19b.
The above-explained conventional structure performs the correction in a condition determined corresponding to the shading distortion at the manufacture of the image processing apparatus, and is unable to achieve correct correction and result in a reading error in case the state of shading varies from the initial state corresponding to the correcting information, for example due to a change in the intensity distribution of the light source 2 resulting from a time-dependent deterioration thereof, or dust or smear on the mirror 7, lens 9 or image sensor 11, or a deterioration in the sensitivity of the image sensor 11. Also in such conventional structure, the operator is unable to know the cause of a reading error and is therefore unable to take a suitable countermeasure.